Method for forming ordered structure of fine metal particles

ABSTRACT

In order to preventing thiol-coated metal particles from being liberated from a self-aligning membrane on a substrate during coating the metal particles deposited on the self-aligning membrane with thiol molecules, this invention provides a process for forming a metal particle ordered structure wherein a voltage is applied on the substrate for preventing the metal particles from being liberated from the self-aligning membrane during coating the metal particles deposited on the self-aligning membrane on the substrate with thiol molecules.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for treating metal particlesdeposited on a substrate with an organic solvent. In particular, itrelates to a process for forming a two-dimensional ordered structure ofmetal particles deposited on a self-aligning membrane.

2. Description of the Prior Art

A single-electron device controls electrons to a single electron levelutilizing a “coulomb blockade” phenomenon where coulomb energy inhibitstunneling of electrons. A single-electron device has the followingexcellent properties.

(a) A temperature at which the device can operate increases as its sizeis reduced.

(b) A conductor or semiconductor material can be used.

(c) Since it can control a single electron, a power consumption is alsoreduced to one several ten thousandth compared to that for aconventional device.

In order to operate such a single-electron device at a room temperature,its capacitance must be a level of 10⁻¹⁸ F., which indicates a devicesize of several nanometers. There have been various attempts toconstruct an ordered structure with such a size, but no satisfactoryprocesses have not been discovered.

The following two types of processes have been mainly studied forconstructing an ordered structure in a nanometer level.

(i) Processes employing conventional lithography; and

(ii) Processes forming a fine structure in a self-aligning manner.

The lithography technique described in (i) has been developed to a levelthat a pattern with a size of about {fraction (1/10)}μm can beprocessed. It, however, is not adequate for processing with goodreproductivity in a several nm level. Drawing with a convergent electronbeam has been mainly investigated. However, drawing with an electronbeam, in which each pattern must be drawn with an electron beam, hasdrawbacks, e.g., that a throughput may be reduced and that an orderedstructure formed may be damaged during electron-beam drawing.

Much attention has been paid to the process in (ii) above forming anordered structure in a self-aligning manner, which depends on advance instudy for metal particles with a size range of several nm to several tennm and their agglomerates (clusters) in the fields of organic chemistryand catalyst chemistry.

An ordered structure used for a single-electron device may be made of aconductor in a nanometer level. Thus, this process applies metalparticle agglomerates with a size of several nm to a single-electrondevice. A particularly noticeable process is that metal particles arecoated with thiol molecules to form metal thiol particles.

Metal particles with a size of several nm are highly reactive because ofa larger ratio of a surface area to a volume so that they mayagglomerate when being left as they are. Metal particle surfaces are,therefore, coated with thiol molecules to prevent the metal particlefrom agglomerating.

Metal particles may be deposited on a substrate by first dropping asolution of metal thiol particles and then drying it. This process,however, has a drawback that the number of metal particles constitutingan agglomerate (cluster) and pattern forming of an agglomerate (cluster)cannot be controlled.

A further advanced process is, for example, that metal particles aredeposited on an SAM made of organic molecules; and the metal particlesare treated with thiol and then dispersed on the SAM to form an orderedstructure as described in T. Sato, D. Brown, F. G. Johnson, Chem.Commun., 1007 (1997).

The process will be described with reference to FIG. 3, in which a metalemployed is gold.

On a silicon oxide film is formed an SAM using APTS(3-(2-amino-ethylamino)propyltrimethoxysilane). The substrate isimmersed in a solution containing gold particle colloid (gold colloidsolution) separately prepared to deposit the gold colloid on the SAM.Then, a loose electrostatic bond is formed between the amino group inAPTS and the surface of the gold colloid so that the gold colloid isfixed on the SAM (FIG. 3(c)).

The term “gold colloid” as used herein refers to each gold colloidparticle.

Gold colloids have a positive charge and thus repulse each other, sothat they exist apart from each other by a distance depending on acharge on the colloids, without forming an agglomerate (cluster). Atthis stage, gold colloids cannot be bound each other to form anagglomerate (cluster). Furthermore, the maximum number for gold colloidper a unit area of a substrate on which they are loaded is limited (FIG.3(a)).

After the treatment, the SAM on which gold colloids are loaded isimmersed, together with the substrate, in a thiol solution to coat themetal particle surface with the thiol molecules.

A sulfur atom in a thiol molecule is very apt to be bound to gold.Specifically, a thiol molecule cleaves a loose electrostatic bondbetween a gold colloid and the amino group in the SAM to form a covalentbond with gold. Thus, gold particles are coated with thiol molecules toprovide gold thiol particles (FIG. 3(d)).

Once coated with thiol molecules, gold thiol particles can be dispersedon the SAM surface. Thus, the gold thiol particles are dispersed whilebeing bound each other via van der Waals force to form a two-dimensionalordered structure when being in contact with each other (FIG. 3(b)).

According to this conventional process, silver or platinum can be usedas a metal to provide a two-dimensional ordered structure on an SAM.

As described above, this process is adequately sophisticated to providea metal particle ordered structure with a size of several nm with goodreproductivity.

A cycle of metal particle adhesion and thiol treatment may repeated toincrease the number of metal particles on the SAM surface for providinga two-dimensional ordered structure with an extent comparable to thesubstrate area.

The process of the prior art has a drawback that metal particles loadedon the SAM are detached from the SAM during thiol treatment. Although itdepends on some factors such as treatment conditions and the size of themetal particles, all metal particles loaded may be sometimes detached.It is because a “bond between a thiol molecule and a metal particle” isformed during thiol treatment so that an electrostatic bond between ametal colloid and an amino group in the SAM is cleaved. Then, metalparticles which have been bound to the SAM surface are liberated so thatthey can be dispersed on the SAM. At the same time, an electrostaticbond is, however, lost so that a force binding metal thiol particles onthe SAM becomes weaker and the metal thiol particles begin to beliberated in the solution.

Using gold as a metal, liberation of gold thiol particles from an SAMwas significant when the size of the metal particles was less than 10nm. Furthermore, when the size of the metal particles was 1 nm or less,metal particles little remain on a substrate. It was, therefore, verydifficult to form an ordered structure.

FIGS. 3(a) and (b) schematically show such states. It is assumed that107 metal particles exist on an SAM before thiol treatment. After thioltreatment, an ordered structure in which the metal thiol particlesagglomerate is formed on the SAM while about a half of the metal thiolparticles are detached from the SAM, so that only 63 particles remain.

SUMMARY OF THE INVENTION

In view of these problems, an objective of this invention is to preventmetal thiol particles from being detached from an SAM during coatingmetal particles deposited on the SAM with thiol molecules.

Another objective of this invention is to effectively form a metalparticle ordered structure on an SAM without losing metal particles. Afurther objective of this invention is to provide a process for forminga metal particle ordered structure in which the size of the orderedstructure of metal particles is adjustable.

This invention proposes a process for forming a metal particle orderedstructure on a substrate, comprising the steps of:

(1) immersing a metal oxide film substrate in a solution containing atleast APTS (3-(2-amino-ethylamino)propyltrimethoxysilane) to form aself-aligning membrane (SAM) of APTS on the substrate surface;

(2) immersing the substrate with the SAM in a solution containing metalparticle colloid with a particle size (D) of 0.8≦D≦10 nm to load metalcolloids on the SAM surface;

(3) immersing the substrate on whose SAM surface metal colloids areloaded, in a solution containing at least a material having a thiolgroup to thiolate the metal particles for providing metal thiolparticles; and

(4) applying a given voltage between the substrate where metal colloidsare loaded on the SAM surface as the first electrode and the secondelectrode in the solution while conducting the above step (3).

According to this invention, when treated metal colloids pre-loaded onan SAM with a thiol, a voltage can be applied between a substrate and athiol solution to prevent metal thiol particles from being liberatedfrom the SAM by the action of electrostatic attraction between the metalthiol particles and the substrate.

According to the prior art, as the size of metal particles decreases,after the thiol treatment the amount of the particles liberated from theSAM increases. It has been, therefore, very difficult to keep metalparticles with a small size on the SAM. When using, for example, gold asa metal, gold particles with a size of 1 nm or less cannot bepractically kept on an SAM.

According to this invention, however, even gold particles with a size of1 nm or less it becomes possible to be easily kept gold thiol particleson an SAM.

In this invention, metal particles liberated from an SAM are less thanthe prior art. This invention can, therefore, eliminate loss of metalparticles to effectively form a metal particle ordered structure.

A voltage applied between the substrate and the thiol solution may bevaried to adjust an effective moving length of the metal thiol particleson an SAM and thus to easily control the size of the metal particletwo-dimensional ordered structure. The term “effective moving length” ofmetal thiol particles as used herein means a moving distance per a unittime of the metal thiol particles on an SAM, indicating diffusibility ofthe metal thiol particles on the SAM. A longer effective moving lengthindicates that metal thiol particles can be more easily diffused on anSAM.

Specifically, a lower applied voltage during thiol treatment allowsindividual metal thiol particles to more easily move on an SAM, i.e., alonger effective moving length. A probability of collision between metalthiol particles is, therefore, increased. In this case, a largetwo-dimensional ordered structure may be easily formed even with a smallnumber of particles.

On the other hand, a higher applied voltage restricts movement ofindividual metal thiol particles on an SAM, i.e., a shorter effectivemoving length. A probability of collision between metal particles on thesubstrate is, therefore, reduced so that a small two-dimensional orderedstructure may be formed even with a large number of particles.

An effective moving length depends on the size of metal particles. Ifthe other conditions are identical, a larger particle size gives asmaller effective moving length of metal thiol particles.

As described above, this invention allows the size of a two-dimensionalordered structure and its surface density to be controlled via the sizeof metal particles and an applied voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-FIG. 1D schematically show forming a metal particle orderedstructure according to this invention.

FIG. 2 schematically shows voltage application during thiol-moleculetreatment according to this invention.

FIG. 3A-FIG. 3D schematically show deposition of metal particles andforming an ordered structure according to the prior art.

In FIG. 1C, FIG. 1D and FIG. 2, 11 is an SAM; 12 is a gold colloid; 13is a substrate; 15 and 23 are power supplies; 16 is a gold thiolparticle; 21 is a counter electrode; 22 is a substrate; and 24 is analcohol solution of a thiol.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A suitable substrate for forming an SAM may be a metal oxide film havinga hydroxyl group for reacting with APTS on its surface, including atitanium dioxide film, a nickel oxide film, an alumina film, quartz andglass. Silicon oxide is particularly suitable because a silicon singlecrystal with good quality is readily available.

Metal particles used in this invention preferably have a particle sizewithin a range of 0.8 nm to 10 nm.

Without voltage application, an electrostatic force binding metal thiolparticles to an SAM is substantially equivalent to ambient thermalfluctuation so that the metal thiol particles may be detached from theSAM by small stimulation.

A voltage may be applied during thiol treatment, as in the presentinvention, to reinforce an electrostatic force maintaining a bondbetween an SAM and a metal thiol particle so that metal thiol particlesmainly comprised of those with a size of 10 nm or less, which may beliberated in the prior art, can be bound on the surface.

The lower limit of 0.8 nm is a temporary value, which is the minimumsize of metal particles which we could obtain (Nanoprobes, Inc., goldparticles, an average particle size of 0.8 nm). According to ourevaluation, no gold particles were not liberated from an SAM even forgold particles with a size of 0.8 nm. It may indicate that thisinvention is effective in preventing gold particles with a size of lessthan 0.8 nm from being liberated from an SAM.

A thiol molecule for coating to form metal thiol particles may beselected from those having a saturated aliphatic hydrocarbon or aromatichydrocarbon chain, including dodecanethiol, hexanethiol andbenzenethiol. Our investigation results indicate that as the length of ahydrocarbon chain in the thiol increases, a higher voltage is requiredto restrict metal thiol particles on an SAM. According to ourevaluation, for the purpose of restriction of metal thiol particles on asubstrate, the carbon number may be desirably 8 to 14 when using astraight saturated aliphatic hydrocarbon chain.

The chain length of a thiol molecule is also related to diffusibility ofmetal thiol particles coated with thiol molecules on an SAM, i.e.,tendency to detachment from the SAM.

We have studied the carbon number in a thiol molecule in terms ofdiffusibility on an SAM, using gold particles with a size of 2 to 3 nmwhich are most promising for preparing a single-electron device, andhave obtained good results with the carbon atom number of 10 to 12. Goodresults herein mean that gold particles were not liberated from an SAMwhile a gold particle ordered structure was rapidly formed on the SAM.

The carbon number in a thiol molecule defines a distance between metalparticles in a two-dimensional ordered structure, which is a determinantfactor for a tunnel barrier height in a single-electron device.

For the purpose of a tunnel barrier in a single-electron device, adesirable distance between metal particles is 1 nm or less. A chainlength of a thiol molecule giving such a distance experimentallycorresponds to a thiol molecule having 10 to 12 carbon atoms.

However, as described above, the evaluation results are limited to goldparticles with a size of 2 to 3 nm. It is, therefore, desirable toconduct reevaluation under the conditions when the size or the type ofmetal particles is varied.

A solvent for dissolving thiol molecules is suitably ethanol.

Binding strength of a thiol molecule on the surface of a metal particlesignificantly depends on a concentration of a thiol solution. Strictlyspeaking, a desirable concentration of a thiol solution is 0.1 to 10mmol/L when using dodecanethiol as a thiol molecule, probably dependingon a concentration of metal particles in the solution and the type ofthe thiol molecules used.

When a concentration of the thiol solution is 0.1 mmol/L or more, theamount of the thiol molecules is adequate to coat metal particles,providing metal particles completely coated with thiol molecules. On theother hand, a concentration of 10 mmol/L or less may prevent micelleformation of the thiol molecules, due to which reactivity of themolecules with metal particles may be reduced.

The term “self-aligning membrane” as used in Claims refers to a membranewhere the surface of a substrate is coated with a unimolecular layer ofcompound which has, in one end, a functional group which can react toform a bond with an atom in the substrate surface, and has, e.g., analkyl-chain backbone. It is characterized in that since the functionalgroup binds to an atom in the substrate surface, a lattice period inmolecule adsorption is an integral multiple of that for atoms in thesubstrate surface and molecules are oriented to a given direction fromthe substrate surface by interaction between alkyl chains in thebackbone. When the substrate surface is coated with the molecules, noexposed atoms are present in the substrate surface, and then depositionof the molecules constituting the self-aligning membrane comes to endafter forming a unimolecular layer.

The term “ordered structure” as used herein refers to a structure formedby two-dimensional agglomeration of metal particles on an SAM.

The above given voltage may be desirably such that the metal thiolparticles can be diffused on the self-aligning membrane while the metalthiol particles are not detached from the substrate surface.

The configuration of this invention is characterized in that effect ofinhibiting detachment of metal thiol particles is independent ofpolarity of a voltage applied to an electrode. Equivalent effect may beobtained whether the substrate is an anode or cathode during voltageapplication. However, as an applied voltage is increased, liberation ofmetal particles from an SAM becomes more difficult, which indicates thata force acting on the metal particles may be electrostatic.

It may be because of some mechanism by which metal thiol particles arealways charged in a polarity opposite to that of a voltage applied tothe substrate, but details are still unclear.

A voltage applied is desirably at least sufficient to prevent metalthiol particles with a size within the above range from being detachedfrom an SAM surface, while it is desirably up to a level sufficient toallow the metal thiol particles to be two-dimensionally diffused on theSAM even when a voltage applied is large.

An applied voltage may be suitably selected, depending on some factorssuch as the size of metal particles, the desired size of atwo-dimensional ordered structure and the type of thiol moleculescovering the metal particles.

An applied voltage is desirably within a range of 0 to 5 V.

When an applied voltage is 5 V or less, even long-term voltageapplication may not cause adhesion of probably organic contaminants onan SAM.

It has been confirmed that an applied voltage within the above range iseffective to metal thiol particles in which gold particles with a sizeof 0.8 to 10 nm are coated with thiol molecules.

This invention proposes a process for forming a metal particle orderedstructure further comprising the step of immersing the substrate havinga metal particle ordered structure formed by the above process forforming a metal particle ordered structure in a solution of a compoundhaving at least two thiol groups to bind metal thiol particles together.

A two-dimensional ordered structure of metal thiol particles ismaintained by van der Waals force between metal thiol particles. Sincevan der Waals force is not so strong, small stimulation may disturb thetwo-dimensional ordered structure when the metal thiol particles have alarge size.

When a substrate after forming a two-dimensional ordered structure isimmersed in a solution containing polyfunctional thiol molecules havingat least two thiol groups as in this invention, a part of themonofunctional thiol molecules bound to metal thiol particles arereplaced with the polyfunctional thiol molecules so that metal thiolparticles are covalently bound each other, forming a more rigidtwo-dimensional ordered structure.

Suitable polyfunctional thiols which can be used include hexanedithioland octanedithiol.

This invention also provides a process for forming a metal particleordered structure comprising the steps of placing the first electrode ina solution containing metal thiol particles in which the surface ofmetal particles with a size D of 0.8≦D≦10 nm are coated with a substancehaving at least one thiol group; placing, as the second electrode, insaid solution an oxide substrate comprising a self-aligning membranemade of APTS on its surface for deposition of the metal particles; andapplying a voltage between the electrodes to deposit the metal thiolparticles in the solution on the substrate.

As described above, a solution containing metal particles coated withthiol molecules may be prepared in advance; a substrate as the firstelectrode and the second electrode may be immersed in the solution; andthen a voltage may be applied to deposit on the substrate metalparticles coated with thiol molecules near the substrate to form atwo-dimensional ordered structure.

The above metal particles may be selected from gold, platinum and silverparticles.

An embodiment of this invention will be described with reference to FIG.1A-FIG. 1D. In this embodiment, gold is used as a metal particle. FIG.1A-FIG. 1D schematically show a process for thiol treatment whileapplying a voltage to gold colloids deposited on a substrate. An SAMmade of APTS 11 having a silane group at one end and an amino group atthe other end is formed on a substrate 13 of an insulative silicon oxidefilm. First, the substrate is immersed in a solution containing goldcolloids so that a gold colloid 12 forms an electrostatic bond with theamino group in the SAM 11 (FIG. 1A and FIG. 1C). Then, the substrate onwhich gold colloids 12 are deposited is treated with a thiol solution.The second electrode is placed in the thiol solution, a voltage 15 isapplied to the substrate and then the substrate is immersed in the thiolsolution (FIG. 2). The gold colloids 12 are coated with thiol molecules14 to provide gold thiol particles 16, which have reduced adsorptivepower with an amino group and thus may be diffused on the surface of theSAM 11 (FIG. 1D). On the other hand, gold thiol particles 16 are notliberated from the SAM 11 owing to electrostatic attraction by theapplied voltage. Thus, gold thiol particles may freely move on the SAM11 and, when colliding each other, can form a two-dimensional orderedstructure by van der Waals bonding (FIG. 1B).

Effects of this invention are schematically shown in FIG. 1A and FIG.1B. For example, a substrate in which 107 gold colloids are loaded on anSAM is treated with a thiol while applying a voltage, to coat thesurface of the gold particles with the thiol to form gold thiolparticles. Thus, gold thiol particles are allowed to be diffused on theSAM to form a two-dimensional ordered structure. In the prior art shownin FIG. 3A-FIG. 3D, many of gold thiol particles are liberated from anSAM during thiol treatment, whereas according to this invention, no goldparticles are lost after thiol treatment.

This invention will be more specifically described with reference to,but not limited to, examples.

EXAMPLE 1

With reference to FIG. 3A-FIG. 3D and FIG. 2, an embodiment of a metalparticle ordered structure formed by a process of this invention will bedescribed.

(1) Preparation of a Substrate

A silicon oxide film was used as a substrate 13 for forming aself-aligning membrane by APTS. The film was prepared by thermallyoxidizing the surface of the silicon substrate 13 to 200 nm. In thelight of relationship between cleanliness of the substrate andadhesiveness of an SAM, the surface of the substrate was cleaned by O₂plasma treatment.

(2) Forming an SAM on the Substrate

The substrate 13 whose surface was cleaned was immersed in an aqueoussolution containing APTS (3-(2-amino-ethylamino)propyltrimethoxysilane)at 5 mmol/L for 30 min.

The surface of the silicon oxide film had many hydroxyl groups, whichwere bound to a silane in APTS to form an SAM 11 on the metal oxidefilm.

Then, the substrate 13 was washed with pure water and dried by N₂ gasblowing to complete bond formation between the substrate 13 and APTS.

(3) Preparation of a Gold Colloid Solution

A metal used was gold in this example.

Gold colloids was supplied as an aqueous gold colloid solution with aparticle size of 5 nm (British Bio Cell Inc.).

(4) Loading of the Gold Colloids on the Substrate

The substrate 13 prepared in step (2) was immersed in the gold colloidsolution from step (3) for 30 mm, whereby gold colloids 12 were adsorbedon the SAM by forming a weak electrostatic bond with an amino group.Gold colloids 12 had the same polarity so that they repulsed each otherby electrostatic repulsion and thus dispersedly adsorbed on the SAM 11while being separated each other at a certain distance (FIG. 1A).

At the end of this step, the number of gold colloid on the surface ofthe SAM 11 was determined by scanning electron microscopy (hereinafter,referred to as “SEM”). Then, the number of gold colloid particles per aunit area was 2000/μm².

(5) Thiol Treatment

Thiol treatment was conducted for coating the gold colloids with thiolmolecules 14. In this treatment, a counter electrode 21 was placed asthe second electrode as shown in FIG. 2 for preventing liberation of thegold thiol molecules from the surface of the SAM 11 and then thesubstrate was immersed in a thiol-alcohol solution 24 (a 5 mmol/Lsolution of dodecanethiol in ethyl alcohol) while applying a voltage 23to the whole substrate 22 for 2 hours. During the process, a voltageapplied was 3 V. Although a voltage was applied using the substrate 22as an anode, the substrate 22 may be, on the contrary, used as acathode.

As a result, gold thiol particles are allowed to move on the surface ofthe SAM 11 by weak electrostatic attraction derived from an appliedvoltage, but are so firmly fixed on the substrate surface that theycannot be liberated from the surface of the SAM 11. A gold thiolparticle is diffused on the surface of the SAM 11 and, when being incontact with another gold thiol particle, form a bond via van der Waalsforce to form an ordered structure.

At the end of step (5), the ordered structure in the SEM was observed.It was then found that ordered structures in which 15 to 20 gold thiolparticles were two-dimensionally agglomerated were evenly distributed onthe SAM 11 at a surface density of about 100/μm².

The size of a gold thiol particle ordered structure can be adjusted byan applied voltage. Increase in an applied voltage causes reduction inthe number of gold thiol particles contained in an ordered structure,leading to reduction in a size of the ordered structure. On the otherhand, increase in an applied voltage causes increase in the number ofgold thiol particles contained in an ordered structure, resulting inincrease in a size of the ordered structure.

When an applied voltage was increased to 5 V, the number of gold thiolparticles was 10 to 15, which was smaller than when an applied voltagewas 3 V, while a surface density of the ordered structure was increasedto 200/μm².

On the other hand, when an applied voltage was reduced to 1 V, so thatthe number of gold thiol particles contained in an ordered structure wasincreased to 30 to 40, which was larger than when an applied voltage was3 V, while a surface density of the ordered structure was reduced to30/μm².

(6) Increase in the Number of Gold Particles on the Substrate

This step may be conducted when forming an ordered structure having alarge surface, and may be omitted if a gold thiol particle orderedstructure can be formed with a desired size and a desired density instep (5).

The steps (4) and (5) can be repeated to increase the number of goldthiol particles on the substrate.

EXAMPLE 2

In an ethanol solution containing octanedithiol at 5 mmol/L was immerseda substrate prepared as described in Example 1 and having atwo-dimensional ordered structure of gold thiol particles whose surfacewas coated with thiol molecules. By this treatment, a part of the thiolmolecules coating the gold thiol particles were replaced and gold thiolparticles in contact with each other were bound. Thus, thetwo-dimensional ordered structure on the substrate surface formed inExample 1 could be further reinforced.

EXAMPLE 3

A substrate comprising an SAM on whose surface gold colloids are loadedwas prepared as described in steps (1) to (4) of Example 1.

(5) Treatment with a Difunctional Thiol

Unlike Example 1, the substrate prepared in step (4) was immersed in asolution of a difunctional thiol. A difunctional thiol used was a 5mmol/L solution of octanedithiol in ethanol.

The substrate was used as an anode while a counter electrode was placedas the second electrode in the solution of octanedithiol in ethanol, andthen a voltage of 3 V was applied between the electrodes for 2 hours.

As a result, SEM observation indicated that ordered structures in which15 to 20 gold thiol particles were two-dimensionally agglomerated wereevenly distributed on the SAM surface at a surface density of about100/μm².

The two-dimensional ordered structure of gold thiol particles obtainedin this example is very rigid and highly resistant to, e.g., heatingbecause gold thiol particles are covalently bound each other.

EXAMPLE 4

A substrate comprising an SAM on its surface was prepared as describedin steps (1) and (2) of Example 1.

(3) Preparation of a dispersion of Gold Thiol Particles

Commercially available gold particles with a size of 5 nm were dispersedin a 5 mmol/L solution of 1-dodecanethiol in ethanol to prepare adispersion of gold thiol particles whose surface were coated with athiol in advance.

(4) Placing Electrodes

A ring counter electrode 21 was placed as the second electrode in thedispersion of gold thiol particles prepared in step (3), as shown inFIG. 2. Then, the above substrate 22 comprising an SAM on its surfacewas immersed in the dispersion of gold thiol particles 24.

(5) Voltage Application

A DC power source 23 was connected such that the counter electrode andthe substrate became an anode and a cathode, respectively, and then avoltage of 3 V was applied.

(6) Formation of an Ordered Structure

At the end of voltage application for about 10 hours, the substrate wasremoved and SEM observation for the surface of the SAM film indicatedthat ordered structures in which 20 to 30 gold thiol particles wereagglomerated were formed at a surface density of 50/μm².

COMPARATIVE EXAMPLE 1

Ordered structures of gold thiol particles were formed on an SAM asdescribed in Example 1, omitting voltage application during thioltreatment in step (5).

At the end of steps (4) and (5) as described in Example 1, SEMevaluation indicated that after thiol treatment, gold thiol particles onthe SAM were reduced to 50% of the number before treatment, according tothe prior art.

As described above, this invention can prevent metal thiol particlesfrom being liberated from an SAM during coating of metal particlesdeposited on a substrate with thiol molecules. A voltage applied duringthiol treatment may be adjusted to control the size of a metal particleordered structure.

Furthermore, a metal particle ordered structure can be effectivelyformed on an SAM without losing metal particles.

What is claimed is:
 1. A process for forming a metal particle orderedstructure on a substrate, comprising the steps of: (1) immersing a metaloxide film substrate in a solution containing at least APTS(3-(2-amino-ethylamino)propyltrimethoxysilane) to form a self-aligningmembrane (SAM) of APTS on the substrate surface; (2) immersing thesubstrate with the SAM in a solution containing metal particle colloidwith a particle size (D) of 0.8≦D≦10 nm to load metal colloids on theSAM surface; (3) immersing the substrate on whose SAM surface metalcolloids are loaded in a solution containing at least a material havinga thiol group to thiolate the metal particles for providing metal thiolparticles; and (4) applying a voltage between the substrate where metalcolloids are loaded on the SAM surface as a first electrode and a secondelectrode in the solution while conducting the above step (3).
 2. Theprocess for forming a metal particle ordered structure as claimed inclaim 1 wherein the applied voltage is 0 to 5 V.
 3. The process forforming a metal particle ordered structure as claimed in claim 1,further comprising after step (3) immersing the substrate in a solutioncontaining a substance comprising a thiol having at least two thiolgroups to bind metal thiol particles together.
 4. The process forforming a metal particle ordered structure as claimed in claim 1 whereina certain level of voltage is applied such that the metal thiolparticles are not liberated from the substrate surface while they can bediffused on the self-aligning membrane.
 5. The process for forming ametal particle ordered structure as claimed in claim 4 wherein theapplied voltage is 0 to 5 V.
 6. A process for forming a metal particleordered structure comprising the steps of placing a first electrode in asolution containing metal thiol particles in which the surface of metalparticles with a size D of 0.8≦D≦10 nm are coated with a substancehaving at least one thiol group; placing, as a second electrode, in saidsolution a substrate comprising a self-aligning membrane on its surfacefor deposition of the metal particles; and applying a voltage betweenthe electrodes to deposit the metal particles in the solution on thesubstrate.
 7. The process for forming a metal particle ordered structureas claimed in any one of claims 1 to 6 or 5 wherein the metal particlesare gold particles.
 8. The process for forming a metal particle orderedstructure as claimed in any one of claims 1 to 6 or 5 wherein the metalparticles are platinum particles.
 9. The process for forming a metalparticle ordered structure as claimed in any one of claims 1 to 6 or 5wherein the metal particles are silver particles.